Recovery of hydrogen fluoride

ABSTRACT

The disclosure 113 of methods for recovering hydrogen fluoride from aqueous feed liquids containing fluosilicic acid in solution. The fluosilicic acid is converted to 07161986 hydrogen fluoride, which is reclaimed by absorption and desorption, using sodium fluoride-sodium bifluoride in fluidized beds.

United States Patent 1191 Hartig 1 lJan. 16, 1973 RECOVERY OF HYDROGEN[56] References Cited FLUORIDE UNITED STATES PATENTS [76] lnvemo HmigGulf Design 2588 786 3/1952 Wlntcr ..23 153 and Engineering Corp 1245011111 2:966:354 8/1961 La Croix lngraham nu Lakeland, 3,087,787 4/1963Flemmert.... 33801 3,140,152 7/1964 Rucker 3; 157,469 11/1964 TuftsNot1ce: The portion of the term Of thlS 3 r64 44o Levey Jr H Y patentsubsequent to J ly 16, 1986. 3,207,579 9/1964 Burkh ardt... has beendisclaimed. 3,219,410 11/1965 Dexter ..23/88 X [22] Flled: March 1965Primary ExaminerEdward Stern Appl. No.: 437,110

US. Cl. ..423/483, 423/336, 423/337, 1 423/462, 423/472, 423/489 Int.Cl. ..C01b 7/22 Field of Search ..23/153, 88, 1F, 1 FT, 205, 23/182 VAttorney-Carl B. Fox, Jr.

[57] ABSTRACT 2 Claims, 3 Drawing Figures United States Patent 1 1Hartig 1 *Jan. 16, 1973 PATENTEUJAH 16 I975 SHEEI 1 [IF 3 ownPATENTEDJAN Is 1915 SHEET 3 [1F 3 RECOVERY OF HYDROGEN FLUORIDE Thisinvention pertains to the recovery of hydrogen fluoride (HF) from gasand liquid streams. According to the process herein disclosed, the gasor liquid stream containing fluorine is first subjected to treatments toprovide a gasstream containing fluorine in the form of HF. Thesetreatments will vary depending upon the form of the gas or liquid streamto be processed. After all, or substantially all, of the fluorinecomponents have been converted to HF, the HF is removed by absorption ina fluid bed constituted by powdered, granular, or pelletized sodiumfluoride (NaF). The sodium fluoride combines with HF and is converted tothe bifluoride (NaI-IF I-IF is recovered by decomposition, ordesorption, of the HP from the bifluoride. The sodium fluoride may berecycled for subsequent reuse. The separated HF, in substantiallyuncontaminated condition, is liquefied by condensation and is a highgrade product.

A principal object of the invention is to provide methods for recoveryof hydrogen fluoride from gas and liquid streams.

An additional object of the invention is to provide such processes whichare economical, dependable, and capable of application to diverse feedcompositions.

An additional object of the invention is to provide such processeswherein ordinary inexpensive materials of construction may be used infabrication of the processing equipment.

Other objects and advantages of the invention will appear from thefollowing description of a preferred embodiment thereof, reference beingmade to the accompanying drawings, of which:

FIG. 1A, FIG. 1B, and FIG. 1C are sequential portions of a schematicflow diagram illustrating a preferred embodiment of the processesprovided according to the invention.

Referring first to FIG. 1A of the drawings, there is shown a source of awater solution of fluosilicic acid (H SiF which may also contain othermaterials, fluorine-containing or not, in solution or suspension. Thesolution from source 10 is moved through pipe 11 by pump 12 and throughpipe 13 to heat exchanger 14, where the solution is indirectly heated bycountercurrent steam. The heated solution passes through pipe 15 tovapor release tank 16.

Steam supplied by boiler is delivered through pipe 21 and pipe 22 to bemixed with the heated solution in pipe 15 before its introduction intothe vapor release tank.

Branch pipe 24 supplies steam to heat exchanger 14, the condensatetherefrom exiting through pipe 25.

In vapor release tank 16, all materials volatile at the temperaturemaintained in the vapor release tank are expelled therefrom through pipe26. Since the source 10 of aqueous fluorine solution is almostinvariably obtained from a gas scrubbing unit of a phosphate acidulationplant, the solution will usually contain phosphoric acid (H PO The vaporrelease tank evaporation of the liquid feed will concentrate theresidual materials therein. For example, if the feed from source 10contains from I percent to 1.5 percent P 0 (as I'I PO in the solution),then the H PO may be concentrated in the vapor release tank to aconcentration of, say, 40 percent to 50 percent P 0 This concentratedphosphoric acid is delivered from the vapor release tank through pipe17, and is a valuable by-product material, suitable for use infertilizer manufacture, in mineral feed supplement manufacture, (sinceit is substantially defluorinated), or other uses.

Pipe 26 leads to superheater heat exchanger 27. Steam is supplied to thesuperheater through pipe 28 connecting from pipe 21. In the superheater,the steam indirectly heats the vapors issuing from vapor release tank16, these materials including silicon tetrafluoride (SiF hydrogenfluoride (HF), and water vapor. These materials are delivered from thesuperheater through pipe 31 to heat exchanger 32, usually at atemperature of about 260F. In heat exchanger 32, heat is recovered fromhot gas delivered through pipe 33 from a process component further alongin the system providing gases at a temperature of about 1,250F. Thecooled gases which have been indirectly heat exchanged with the flowthrough pipe 31 exit through pipe 34 for reintroduction into the processat a later stage.

The heated gases leave heat exchanger 32 through pipe 37 which deliversthe gases to decomposer 38. A small amount of fluosilicic acid insolution is delivered to decomposer 38 through pipe 40. The source ofthis material will be later explained.

Decomposer 38 includes a burner 42 into which natural gas or other fuelis delivered through pipe 43. Air is supplied to the burner through pipe44 to support combustion of the fuel. In the decomposer, the gas streamentering through pipe 37 and the small amount of liquid feed throughpipe 40 are heated in intimate mixed contact in the flame resulting fromburner 42. The silicon tetrafluoride (SiF and the fluosilicic acid (HSiF are decomposed so that substantially all of the fluorine content isin the form of hydrogen fluoride (HF). This HF gas is delivered fromdecomposer 38 through pipe 47 to electrostatic precipitator 48. Inelectrostatic precipitator 48 the hot gaseous materials are passedbetween oppositely charged electrode elements which causes removal ofsolid materials from the gas stream. These solids are comprisedprincipally of silicon dioxide (SiO which is usually of light density,for example, of about 5-l 5 pounds per cubic foot. This silica productis a salable by-product of the process.

The silica product exits from the precipitator through pipe 50.

The hot gases from which the solids have been precipitated leaveprecipitator 48 through pipe 33, previously mentioned, and after passingthrough heat exchanger 32 are delivered through pipe 34 to scrubber 54.The gases delivered through pipe 34 to the scrubber are usually at atemperature of about 600F. The gases are composed of the combustionproducts resulting from decomposer 48, HF, SiF, and water vapor, theSiF, gas content being relatively low. From scrubber 54 the gas ispassed through pipe 55 to a second scrubber 56. The gases leavingscrubber 56 through pipe 57 pass through clean-up tower 58 and outthrough pipe 59. Water feed to the clean-up tower and scrubbers issupplied through pipe 64 leading from a suitable water source toclean-up tower 58, then the water flows through pipe 65 to sump 66 oftower 56, this liquid being recycled through tower 56 by pump 67 andrecycle line 68. Branch pipe 69 diverts liquid from sump 66 to sump 71of tower 54. Liquid from sump 71 is recycled through tower 54 by pump 72through pipe 73. Pipe 40, previously mentioned, conveys liquid recycledto the decomposer 38.

Referring now also to FIG. 1B of the drawings, pipe 59 delivers thescrubbed gases to the lower portion of absorber tower 75. Tower 75 hastherein afluid bed of sodium fluoride (NaF) which is maintained influidized condition by the gas stream passing upwardly therethrough.This fluid bed is indicated by reference numeral 76, and consists ofsodium fluoride in sub-divided form suitable for fluidization, such as,for example, in powdered, granular, or pelletized form. Instead of NaFin the absorber bed, it is possible to use potassium fluoride (KF),lithium fluoride (LiF) or the rubidium or cesium fluorides, but thesematerials might cause operating difficulties because of differentdecomposition characteristics of the bifluorides, and for other reasons.Data on use of these materials as absorbents may be found in FluorineChemistry, Vol. I, By Simons, Academic Press, Inc., New York, N.Y. 1950,pages 2629.

Dust or fines collected in scrubber 84 will grow in size in the recycledscrubber liquor before they are reintroduced to absorber 75 through pipe94 with the cooling water. In the absorber, substantially all of thehydrogen fluoride contained in the gas stream reacts with sodiumfluoride of the fluid bed to convert sodium fluoride to sodiumbifluoride.

A portion of the fluid bed material is continuously removed fromabsorber 75 through pipe 76. Sodium fluoride is continuously fed intoabsorber 75 through pipe 77. The gases leaving absorber 75 through pipe79 pass through dust cyclone 80 and then through pipe 81 to scrubber 84.Dust recovered in cyclone 80 is delivered through pipe 85 into pipe 77to be returned to absorber tower 75. Alternatively, the dust collectedin cyclone 80 may be passed through a pipe 87 to pipe 88 leading to sump90 of tower 84. Pipe 88 delivers sodium fluoride from pipe 77 into sump90 to be mixed with the scrubbing solution therein. The scrubbingsolution in sump 90 is recycled to tower 84 by pump 91 through pipe 92.A portion of the liquid recycle in pipe 92 is diverted through pipe 94to absorber 75, this liquid,-principally water, being evaporated inabsorber 75 and hence serving to cool the fluid bed in absorber 75.

Instead of utilizing the heat of evaporation of water added through pipe94 to cool the bed in absorber 75, the bed may be cooled by coolingcoils in the bed or in other suitable manner. The temperature of bed 76must be maintained at a temperature below the decomposition temperatureof NaHF and a temperature of about 250F. has been found to be suitable,although lower or somewhat higher temperatures may be employed.

If more efficient dust recovery is required from the gas leavingabsorber 75, an electrostatic precipitator may be used in place of, orfollowing, cyclone 80. The fines may be dissolved, preferably in hotwater, and crystallized toform particles of larger size forre-introduction to the system. These alternatives may be applied at thelocations of the other dust removal cyclones throughout the system, aswell as at this location in the system.

Makeup NaF for the absorber-desorber system may be provided by addingsoda ash (Na C to the scrubber liquor or. to the absorber. The soda ashwill react to form NaF in the system. NaF may be added, but soda ash ischeaper.

Scrubbed gas leaving tower 84 through pipe 96 is delivered through thelower end of tail scrubbing tower 97. Water is delivered from a suitablewater source to tower 97 through pipe 98 and spray manifold 99. Thewater collected in the base of tower 97 is delivered through pipe 101 tosump of tower 84. Weak l-IF solution in pipe 102 is also added to sump90. Effluent gas from tower 97 exits through pipe 102 impelled by fan orblower 103.

Scrubber 84 and tail tower 97 serve the purpose of cleaning up the gasesfrom absorber 75. The small amount of sodium fluoride added to thescrubbing water in tower 84 is highly reactive to residual HF whichmight have passed absorber 75 with the gases.

Sodium bifluoride (NaHF is moved through pipe 76 to desorber 110.Recovered sodium fluoride (NaF) is delivered from desorber through pipe77, previously mentioned. Hot combustion gas is delivered to heatexchanger coil 112 within the lower portion of tower 1 10 through pipe 113 which receives the hot flue.

gases from combustion box 114 to which gas or other fuel is suppliedthrough pipe 115 and combustion air through pipe 116. A fan 117 isprovided to deliver the combustion air to pipe 116 and to the combustionbox. The sodium fluoride and sodium bifluoride in desorber 110 aremaintained in fluidized condition by a gas stream circulated upwardthrough the desorber through pipe 120, dust cyclone 121, pipe 122,blower or fan 123, pipe 124, indirect heat exchanger 125, and pipe 126.The gas from heat exchanger coil 112 exits through pipe 127 to passthrough heat exchanger to indirectly heat the gas stream circulatingthrough the desorber tower 1 10.

The gas stream circulating upwardly through tower 110 is a hydrogenfluoride gas stream. This hydrogen fluoride gas stream is created by thedecomposition of sodium bifluoride to sodium fluoride and hydrogenfluoride in tower 110. The hot gas entering heat exchanger 112 throughpipe 113 maintains a temperature of about 700F in desorber tower 110,this temperature being sufficient to cause decomposition of the sodiumbifluoride to the sodium fluoride. Dust carryover from tower 110collected by cyclone 121 is returned to the tower through pipe 130.While a temperature of about 700F. in tower 110 has been mentioned assuitable, any temperature above the decomposition temperature of sodiumbifluoride may be employed.

Referring now also to FIG. 1C of the drawings, hydrogen fluoride gas iswithdrawn from pipe 122 through pipe 131 and delivered to countercurrentindirect gas cooler 133. Cooling water is delivered to cooler 133through pipe 134 and withdrawn through pipe 135. Condensate created incooler 133 I is delivered through exit pipe 137 to collection tank 138,and then delivered through pipe 140 to pipe 102 carrying the weakhydrogen fluoride recycle stream. This material, which is in very smallamounts, is composed primarily of water condensed from the hydrogenfluoride gas stream and has some hydrogen fluoride dissolved therein.

The function of the scrubbing operation in packed tower 143 is two-fold.First, any water which may have passed the previous units is absorbed bythe downflowing liquid HF. Presence of water in the gas at this pointmay, for example, occur because of malfunction of theabsorption-desorption units. Second, any sodiumbifluoride dust passed bycyclone 121' is washed from the gas stream, the bifluoride being atleast somewhat soluble in the HF. Therefore, the purity of the HF gasstream is improved by this scrubbing operation.

The scrubbed, cooled hydrogen fluoride gas leaving tower 143 throughpipe 149 is delivered to condenser 150. Condensed anhydrous hydrogenfluoride liquid leaves condenser 150 through pipe 152 as product, asmall amount of this anhydrous product being delivered through pipe 145as reflux for tower 143. Cold water is delivered to condenser 150through pipe 153. The cold water return from condenser 150 passesthrough pipe 154 to flashpot 155. Cold water from flashpot 155 passesthrough pipe 156 to refrigerated water storage tank 157. Therefrigerated water in tank 157 is withdrawn by the pump 158 through pipe153. Steam is delivered through pipe 160 branching from steam pipe 21 atboiler to the upper ends of steamjet pumps or ejectors 162, 163. Watervapor resulting from vacuum evaporation of water in flashpot'155 isdelivered by pipe 165 to the jet of steam-jet 162. Cooling water entersejector 162 through pipe 166 and hot,

water leaves ejector 162 through pipe 167 extending to sump 168. A pipe171 extends from ejector 162 to the jet of steam-jet ejector 163.Exhaust steam leaves ejector 163 through pipe 172, also extending tosump 168. Ejectors 162 and 163 form a two-stage ejector system forpulling a relatively high vacuum at flashpot 155. Ejector 163 decreasesback pressure on ejector 162 thereby enabling ejector 162 to perform athigh vacuum efficiency to cause high evaporation, and high coolingcapacity, at flashpot 155.

small amount 6r" non-icondensible gas, including inert gases as well ashydrogen fluoride, is withdrawn from condenser 150 through pipe 174,which delivers the gas to water scrubber 177. Scrubber 177 has sump 178from which recycled scrubbing liquid is delivered to the top of scrubber177 by pump 179 through recycle pipe 180. Pipe 182 branches from recyclepipe 180 and delivers weak hydrogen fluoride solution to pipe 102leading to the sump of scrubber 84. Scrubbed gas leaving scrubber 177through pipe 184 is delivered to secondary scrubber 185 having sump 186,recycle pump 187 and recycle pipe 188. Scrubbed gas ieaving .tower 185is exhausted to the atmosphere through pipe or stack 190. Fresh water isdelivered to sump 186 of scrubber 185 through pipe 192. Scrubbing waterfrom sump 186 is delivered to sump 178 through pipe 193.

The following Table I summarizes the operatin g con ditions for anexemplary operation of the process. In the left hand column of thetable, various locations in the system are indicated by designation ofthe equipment element within which the process material is disposed. Thecompositions, flow rates, and/or other physical characteristics of eachprocess material are indicated in the other columns of the Table.

TABLE I.EXEMPLARY OPERATING CONDITIONS FOR RECOVERY OF HF, HQPO AND S102Std. Chemical Gallons Lbs. Lbs. cubic analysis, Temper- Matorlals inprocess equipment per per per feet per percent by store, elementsindicated below minute minute hour minute weight F.

Source 10 and pipes 11, 13

(Feed to vapor release tank 16) Boiler 20 ouptput: Steam p.s.i.) Pipe28, steam l Pipe 22, steam Pipe 24, steam Pipe 160, steam Pipe 5%: H POiproduct Pipe I (vapoijm.

H102 (solid)... I'lpom:

Water solution li'l". Pipe 33, hot, gas (from proclpitntor 4x) l'lpo 34,cooled hot gas (from pn-i-ipllotor43) (f u, m 5.

Inert tarsus HlFt,

I (vapor). Pipe 50, S102 protlueL llpe (M, ll2()- SlF4 H2O (vapor) e 37:

Gas (feed to decomposer 38) i HF TABLE l.--(onlinued HM. (Jinnah-ill(inllons libs. Lbs. cubic zumlysls, 'lmnpl-r- Materials in processequipment poi per pm foot per permit", by nun-o, elements indicatedbelow mlnuto minute hour minute weight i Pipe 59: 4

Inert gases HF H1O (vapor) Pipo g1, gas food to scrubber 8 Pipe 76,NnllF 2 (food to desorber 110) (HF) lipi- 11R, N20 (to tailtower-117).... Pipe 10;, gas (exhausted to tl1,lllS1 ll0ll) lipv 113,combustion gas (to coil 112 of desorber 110)-.. Pipe 127, combustion gas(from coil 112) Combustion gas (leaving heat exchanger). Pipe 126, HFrecycle to desorber 110. Pipe 13], HF gas (to cooler 133).- Pipe 142, HFgas (to tower 143). Pipes 140, 182, 102, weak HF solution Pipe 149, HFgas (to condensor 150).. Pipe 152, HF liquid product H2O Pipe174,1nertgas Pipe 153, cold water to condenser 160....

1 Ambient or above. 2 AS P205.

Operating efficiency, with careful control, can approach 100 percent, asindicated in Table l. in normal plant operation, hydrogen fluoriderecovery efficiency can probably be maintained at 98 percent, or higher.Even with poor control, efficiencies of better than 90 percent hydrogenfluoride recovery can most certainly be maintained.

It will be realized that the method need not be commenced with thefluosilicic acid solution indicated at in the drawing. Instead, a gasstream containing fluosilicic acid, silicon tetrafluoride, HF and/orfluorine in other forms may be introduced, for example, at pipe 37leading to decomposer 38, whichmay be the initial elements of theapparatus for performance of the method. In other words, the apparatusindicated at the left hand part of FIG. 1A may not be employed, and theprocess may commence at decomposer 38. Alternatively, the process may beused commencing at absorber 75 w re a gas stream containing HP in vaporform is ao'a' le for treatment. In each of these cases, the HF isrecovered by absorption in the fluid bed 76 and HF recovery at desorber110. Both of these fluid bed units are of new and novel performance, andthe power and raw material requirements thereof are relatively small.

The process as described is economical m use as well as in constructionof the facilities for performance of the process. Most of the equipmentmay be made of ordinary steel because of the temperatures andcompositions of the materials as conveyed through the process.

While all necessary valves, pumps, material conveying equipment, and thelike, are not shown or described, such are within the capabilities ofthose of ordinary skill in the art, and may be providedin any suitableforms available in the art to provide efficient material handlingcharacteristics in the process and in the physical plant, to insuresmooth, trouble free, efficient operation.

While a preferred embodiment of the process provided by the inventionhas been shown and described,

Gas (feed to absorber 75).

ifiiri modifications thereof may be made by a person skilled in the artwithout departing from the spirit of the invention, and it is intendedto protect by Letters Patent all forms of the invention falling withinthe scope of the following claims.

1 claim:

1. Method for recovering anhydrous liquid hydrogen fluoride (HF) from anaqueous feed liquid containing fluosilicic acid (l-l SiF in solution,comprising treating the feed liquid with steam to produce a first gasstream containing hydrogen fluoride, water vapor and silicontetrafluoride, contacting the first gas stream with a flame to produce asecond gas stream containing hydrogen fluoride, water vapor, and solidsilicon dioxide (Si0 in suspension, precipitating the silicon dioxidefrom the second gas stream, passing the second gas stream upwardlythrough a first fluid bed containing sub-divided sodium fluoride (NaF)whereby the hydrogen fluoride in the second gas stream reacts with thesodium fluoride in the first fluid bed to produce sub-divided sodiumbifluoride (NaHF in the first fluid bed, transferring sodium bifluoridefrom the first fluid bed to a second fluid bed, circulating a stream ofhydrogen fluoride vapor upwardly through the second fluid bed whilemaintaining the second fluid bed at elevated temperature to formsub-divided sodium fluoride in the second fluid bed and to form hydrogenfluoride added to said stream of hydrogen fluoride circulating upwardlythrough the second fluid bed, transferring sodium fluoride from thesecond fluid bed to the first fluid bed, withdrawing a stream ofhydrogen fluoride from the stream of hydrogen fluoride circulat' ingupwardly through the second fluid bed, and cooling and condensing thewithdrawn stream of hydrogen fluoride to produce anhydrous liquidhydrogen

2. Method according to claim 1 wherein the anhydrous liquid hydrogenfluoride produced contains at least 95 percent of the fluorine containedin the feed liquid stream.